Nuclear magnetic resonance (NMR) spectroscopy is a commercially widespread method in MR for characterizing the chemical composition of substances. In MR, the measurement sample which is situated in a strong static magnetic field is generally irradiated by radiofrequency (RF) pulses and the electromagnetic reaction of the sample is measured. Further, it is known in solid-state NMR spectroscopy to rotate an NMR sample tilted at the so-called “magic angle” of approximately 54.74° in relation to the static magnetic field during the spectroscopic measurement (“MAS”=Magic Angle Spinning) in order to minimize line broadening on account of anisotropic interactions. To this end, the sample is inserted into an MAS rotor. MAS rotors are cylindrical tubes which are sealed with one or two caps, the upper one being provided with blade elements (“impeller”). The MAS rotor is arranged in an MAS stator and the MAS rotor is driven for the purposes of the rotation by gas pressure by way of the blade elements. The totality of MAS rotor and MAS stator is referred to as MAS turbine.
The MAS turbine is arranged in an NMR-MAS probe head during the NMR measurement. The probe head comprises a cylindrical shielding tube. Housed therein are RF electronic components, in particular RF coils, and the MAS turbine. With the shielding tube thereof, the probe head is typically inserted from below into the vertical room temperature bore of a superconducting magnet, positioned therein and held therein with hooks, supports, screws or the like. The MAS turbine is then situated precisely in the magnetic center of the magnet.
In addition to solid-state NMR, use may also be made of the dynamic transfer of spin polarization (DNP=dynamic nuclear polarization) technique. The DNP technique requires simultaneous irradiation of a magnetic microwave field at a frequency which is higher than the Larmor frequency of the 1H nuclei by a factor of 660. Electron spins are excited by irradiation of a microwave field at a suitable frequency, whereupon a transfer of electron polarization onto the atomic nuclei of the sample may be brought about on account of spin interactions.
Currently, microwave radiation is irradiated into the MAS stator simply through a round hole in the coil block through the RF coil, optionally with a suitable widening of the coil windings at the center, without further measures being undertaken. However, when using this technique, only a fraction of the introduced power reaches the alternating magnetic field at the location of the sample.
Although the article Nanni et al., Journal of Magnetic Resonance 210 (1), 2011, 16-2 describes individual constituents of a generic apparatus such as lens, coil windings, rotor wall thickness, mirror, etc. per se, it does not describe a suitable combination and common optimization thereof. Moreover, the tunability to different samples is not taken into consideration.
WO 2015/107512 A1 likewise relates to an increase in the millimeter wave field for DNP, but on a static, i.e. non-rotating, basis. The MAS technique is only mentioned as a further goal. Moreover, use is not made of a lens here either; instead, use is made of a resonator-like structure.
WO 2015/175507 A1 discloses an NMR-DNP-MAS probe head for receiving a substantially circular-cylindrical hollow MAS rotor with a sample substance in a sample volume. This MAS rotor may be mounted with pressurized gas in a measuring position within the MAS stator with a device for gas supply and set into rotation about the cylinder axis of the MAS rotor with a pneumatic drive. A hollow microwave guide supplies microwave radiation into the sample volume through an opening in a coil block introduced into the wall of the MAS stator. A microwave lens is arranged between the microwave guide and the sample volume for focusing the supplied microwave radiation onto the MAS rotor. The MAS rotor is surrounded by a solenoid RF coil and a microwave mirror for reflecting the microwave radiation emerging from the microwave guide and passing through the sample volume is provided on the inner side of the MAS stator lying opposite the microwave guide. The field distribution shown in FIG. 10C of that reference implies that the known arrangement produces a tunable resonator. The adjustable mirror shown therein is intended to tune a “cavity mode”. Furthermore, FIG. 10C of said document shows that the wavelength corresponds to approximately twice the rotor diameter. For a 3.2 mm system, this means a vacuum wavelength of approximately 6 mm and hence a frequency of 50 GHz. Actually, said wavelength would be even shorter in matter, and so the illustration in FIG. 10C is physically unrealistic whenever the rotor and the measurement sample have a dielectric constant >1; this can usually be assumed.
While such a strongly resonant structure has the advantage of a high field amplitude, a fundamental disadvantage of such resonators lies in that the high quality which is usually obtainable requires a very narrow frequency bandwidth which reacts very sensitively to external or internal influences and therefore requires very precise tuning.